Sometimes a security patch isn't all it's cracked up to be. The security researcher who first found a vulnerability affecting more than 20 different router models says the patch meant to fix it only hides the initial weakness and doesn't remove it whatsoever.

In December, Security researcher Eloi Vanderbeken discovered a backdoor that would allow an attacker to gain administrative access to various wireless routers. The weakness affected equipment from major router makers including Belkin, Cisco, and Netgear.

Vanderbeken's discovery was confirmed by other researchers and by January companies moved to patch the affected routers and close the backdoor. But Vanderbeken, who works for Syancktiv Digital Security, says at least some of those companies didn't do a very good job. In fact, he suggests in a slide deck recently published online that the backdoor wasn't a bug, but a feature.

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The patch, Vanderbeken says, only hides the backdoor and that a secret "knock"--a specially crafted network packet or unit of data, as Ars Technica explains--could be sent to the router and re-open the vulnerability. The ability to reactivate the vulnerability suggests to Vanderbeken the backdoor was created deliberately.

Vanderbeken carried out his tests on a Netgear router (DGN1000) and has published a proof of concept that others can try out on that model. It's not clear, however, how many other previously affected routers also have a poorly patched backdoor.

Once the backdoor is open again an attacker could send commands to the router and gain administrative access. But the vulnerability doesn't mean someone halfway around the world could hack into your router. The backdoor only opens when the attacker is on the same network as the vulnerable router or remotely from the network's Internet service provider.

Nevertheless, secret backdoors like this are never a good idea. They may be intended to make it easier for, say, support technicians to access a router remotely. But weaknesses inevitably get uncovered allowing others with more malicious intentions to take advantage of them.